This invention relates to heat recovery devices and, more particularly, to devices for recoving heat normally lost from a flue for hot gases from a stove, furnace or the like and recirculating same for use in heating a room or building.
It is well known that the efficiency of furnaces and stoves for heating buildings, particularly those burning fossil fuels and wood, is substantially reduced because of the loss of a large amount of the heat energy in the combustion gases exhausted to the atmosphere through a flue and chimney.
Most modern wood and coal burning stoves are desiged for high efficiency and are substantially air tight so that the only air admitted into the combustion chamber enters through an adjustable draft. This permits combustion to be closely controlled for a more even fire and slower burning. However, the temperature of the combustion gases are somewhat lower than that for older stoves having areas through which uncontrolled amounts of air can enter the combustion chamber. Consequently, when wood is being burned, removal of an excessive amount of heat from the combustion gases as they pass through an exhaust flue can cause the temperature to drop below a point where creosote and/or tars separate and collect on cooled surfaces inside the exhaust flue.
Numerous different types of devices have been proposed for recovering some of the waste heat from the exaust flue and recirculating it directly back into the building or to another part of the heating system. In one type of device, the hot flue gases pass over the outer surfaces of a plurality of transversely extending heating tubes. Ambient air passing through these tubes, either by natural flow or propelled by a blower, is heated and recirculated back into the room. Examples of this type device are disclosed in U.S. Pat. Nos. 2,882,023 (Rizzo), issued Apr. 14, 1959, 4,028,817 (Winstel), issued June 14, 1977, 4,176,709 (Johnson), issued Dec. 4, 1979, 4,235,286 (Behlau), issued Nov. 25, 1980 and 4,363,353 (Pranatis), issued Dec. 12, 1982.
This type device typically includes a relatively large number of small heating tubes which can cause a significant cooling of the flue gases. When a wood containing tars or creosote is being used as the fuel, the flue gases can be cooled to the point where these tars or creosote separate and collect as deposits on the outer surfaces of the heating tubes and the interior of the flue. Such deposits are flammable and can cause hazardous chimney fires. Carbon or soot can also deposit on the heating tubes when fossil fuels and other types of wood are being burned. Both type of deposits can severely restrict the flow of the flue gases and create the potential hazard of the exhaust gases backing up into a room, resulting in asphyziation.
In another type device, air is heated as it passes through a housing surrounding the exhaust flue. Examples of this type device are disclosed in U.S. Pat. Nos. 2,468,909 (Yeager et al.), issued May 3, 1949, and 4,278,126 (Skrzypek), issued July 14, 1981, 4,147,303 (Talucci), issued Apr. 3, 1979, and 4,313,562 (Wite), issued Feb. 2, 1982. This type device usually is substantially less efficient in recovering the waste heat.
Examples of other types heat recovery devices are disclosed in U.S. Pat. No. 4,276,929 (Howard), issued July 7, 1981, and French Pat. No. 871,937, published May 22, 1942. The device disclosed in the first patent is quite complicated and bulky and requires a relatively large outer chamber including a baffling system for directing air in opposite directions through two different sections of transversely extending heating tubes in an inner chamber through which the flue gases pass. In the device disclosed in the latter patent, the gases flow through a chamber surrounding an ambient air duct and also through a pipe extending transversely through the air duct.
Prior devices known to applicant including heating tubes arranged for air to pass transversely through the flue gases and some kind of fan or blower to improve heat recovery and recirculation of the recovered heat are relatively complex and require some modification to the existing flue system, making them relatively expensive to manufacture and difficult to install. Also, they are arranged so that, in the event the portion through which the flue gases flow fatigues and/or becomes plugged with carbon or creosote deposits during use, the entire unit must be replaced.